In a hydraulically-driven machine such as a construction machine, the rotational speed of a hydraulic motor, that is, the rotational speed of a cooling fan, is controlled by controlling flow rate of pressurized oil supplied to the hydraulic motor while supplying the pressurized oil discharged from a hydraulic pump for the cooling fan which is driven by an engine to the hydraulic motor which rotates the cooling fan. Then, the control is performed on the rotational speed of the cooling fan so that temperature of cooling water of the engine, temperature of hydraulic oil and the like are to be desired temperature.
A fan rotational speed control method (for example, see Patent Document 1) and the like are proposed as the configuration to control the rotational speed of a cooling fan. FIG. 9 is a flowchart describing a fan rotational speed control method disclosed in Patent Document 1 as being the related art for the invention.
As described in FIG. 9, according to the fan rotational speed control method disclosed in Patent Document 1, control is performed on a pump-motor system so that fan driving is started from a state that the fan rotational speed is at the minimum fan rotational speed Nmin at the time of starting engine (step 1). The pump-motor system is constituted with a hydraulic motor to drive a fan and a hydraulic pump to supply pressurized oil to the hydraulic motor. When the fan rotation is started, control is performed so that the state at the minimum rotational speed Nmin is maintained at least for several seconds (step 2).
After the state of being maintained at the minimum fan rotational speed Nmin at least for several seconds, control to increase the fan rotational speed from the minimum fan rotational speed Nmin gradually is performed (step 3). Then, the pump-motor system is controlled so that the fan rotational speed is increased to the target fan rotational speed Ntf when at least several seconds passes after the fan rotational speed is gradually increased (step 4).
Occurrence of peak pressure and pressure hunting at the pump-motor system is prevented by performing the above control. Accordingly, the pump-motor system is prevented from being broken.